Hydroelectric energy, that transforms the potential energy of water between two levels of a river in kinetic energy and electricity, it is solar and gravitational energy, because the energy that elevates the water is from the sum, through its evaporation, condensation and precipitation in rain or snow.
The natural process of the previous paragraph is capable of being imitated.
It takes advantage from temperature differences between sun areas and shade areas, evaporating, rising and condensing a liquid with intermediate boiling point between the temperature in sun areas and shade areas.
This system is closed and it has devices to control the boiling point, as an internal expansion chamber with adjustable walls or a suction pump.
An advantage of this system is the possibility to store the collected solar energy in gravitational potential energy of the evaporated and condensed liquid.